As increase in definition and narrowing of frames of displays on mobile devices such as mobile telephones have progressed in recent years, many displays having a pixel density of over 400 pixels per inch (ppi) have been made into products. As a key technology for making such displays into products, a so-called monolithic circuit technology in which a driving circuit is formed on a glass substrate has been used. In addition, as back planes (circuit boards) of these, thin film transistor (TFT) circuits which use oxide semiconductors such as indium gallium zinc oxide (In—Ga—Zn—O-based semiconductors; which are oxide semiconductors containing indium (In), gallium (Ga), zinc (Zn), and oxygen (O)) have begun to be used.
Driving circuits formed using monolithic circuit technology are known in which a threshold voltage of a TFT changes depending on elapse of time or temperature. Particularly, the following problems may arise in shift register circuits used as scanning line driving circuits.
A shift register circuit boosts a gate of an output TFT which drives a scanning line to a high voltage using a bootstrap to cause it operate. In order to precharge a gate electrode of the output TFT and prevent leakage during boosting, a diode-connected TFT is used as a set TFT. When such a diode-connected set TFT is used, a precharge voltage of an output TFT has a value decreased by a threshold value of the TFT.
Since TFTs using an oxide semiconductor such as indium gallium zinc oxide have a characteristic that a threshold voltage fluctuates due to voltage stress applied to a gate electrode, characteristics degradation progresses in accordance with elapse of time, and a precharge voltage of a gate electrode of an output TFT due to a diode-connected set TFT decreases accordingly. As the precharge voltage becomes lower, a voltage boosted by a bootstrap also becomes lower, a driving force of the output TFT decreases, and thus output waveforms become blunt. If such degradation develops further, an output voltage becomes lower, and thus a shift register operation becomes unstable.
In addition, a shift register having a bi-directional scanning function in which a shift operation can be performed in both directions by a scanning line driving circuit switching between a forward direction and a backward direction has become necessary. The reason for this is that, for example, the top and the bottom of an image displayed on a display unit are easily reversed when the scanning line driving circuit performs a shift operation in the backward direction.
Patent Document 1 discloses an example of a shift register circuit which enables such a bi-directional shift operation. Each stage of the shift register circuit described in FIG. 2 of Patent Document 1 (which will be referred to as a “unit shift register circuit” also in embodiments of the present invention hereinbelow) has an output TFT (T1), a set TFT (T2), a reset TFT (T3), a set TFT (T4), and a reset TFT (T5). The T2 has a drain terminal and a source terminal in diode connection and is connected to an output of the preceding stage, and the source terminal is connected to a gate terminal of the T1. The T3 has a drain terminal connected to the gate terminal of the T1, a gate terminal connected to an output of the subsequent stage, and a source terminal connected to a VSS (power source voltage VSS) terminal. The T4 has a drain terminal and a source terminal in diode connection that are connected to an output of the subsequent stage, and the source terminal is connected to the gate terminal of the T1. The T5 has a drain terminal connected to the gate terminal of the T1, a gate terminal connected to the output of the preceding stage, and a source terminal connected to the VSS terminal. As a shift register circuit, the unit shift register circuit having the above-described configuration performs a set operation in which the T2 precharges (charges) the gate terminal of the T1 and a reset operation in which the T3 pulls down (discharges) the gate terminal of the T1 in a forward (in the direction from a preceding stage to a subsequent stage) shift operation. On the other hand, in a backward (in the direction from a subsequent stage to a preceding stage) shift operation, a set operation in which the T4 precharges the gate terminal of the T1 and a reset operation in which the T5 pulls down the gate terminal of the T1 are performed. In the unit shift register circuit disclosed in Patent Document 1, the configuration enables a scanning order of gate bus lines to be switched, without using select signals for switching the scanning order.